An optical circulator is a non reciprocal device allowing for the routing of light from one fiber to another based upon the direction of the light propagation. A three port circulator outputs to a second port, light received on a first port and outputs to a third port, light which is input on the second port. The number of ports can be increased arbitrary, and it is possible to have fully circulating devices and quasi-circulating devices where the light from the last port does not return to the first port.
There have been a number of configurations proposed for optical circulators to achieve substantial polarization independence. In particular a circulator can be based on polarization beam spitters (either via walk off crystals or polarization beam splitters) Faraday rotation elements and a combination of optically active elements or birefringent retardation plates. In FIG. 1 an example of prior art is shown, where substantially collimated beams are formed by lenses 101,112,208 and 111. A pair of beam splitting prisms 102 and 107 divide or recombine the light according to the polarization slate. Non reciprocal polarization rotators 103 and 104, and reciprocal rotators 105 and 106 and mirrors 110 and 109 allow polarization independent routing of the light from the lens 101 to the lens 107. Light incident on the lens 107, proceeding from the fiber, is routed to the lens 112 and light incident on 112 is routed to the lens 111. In each case the light can be captured by the optical fiber positioned at the focus. Full description can be found in T. Matsumoto et al. "Polarization-independent optical circulator: an experiment." Appl. Opt. Vol. 19, No. 1 pp. 108-112, 1960. This prior art does not exhibit high isolation and is complicated due to the difficulty of producing and aligning the prisms involved.
It has been shown in the prior art that an isolator can be constructed by appropriate positioning of thin birefringent double refraction crystals in a converging beam as shown in FIG. 2. In this optical isolator, light travelling from left to right as indicated by the upper thick arrow is incident in the fiber 201 and is focused by a lens 202, passing through a flat double refraction crystal 203. The double refraction crystal provides a polarization dependent displacement. Both polarization states pass through a Faraday rotation crystal 204 producing a clockwise rotation of approximately 45 degrees. A crystal 205 provides a reciprocal rotation of the light. A second double refraction crystal 206 allows a second polarization dependent displacement and the images coalesce at the fiber 207. It is necessary to use very thin double refraction crystals, 203 and 206, typically less than 0.5 mm to ensure that low loss is achieved when using single mode fiber to ensure that beam distortion of the extraordinary ray is minimized.
Light travelling from right to left as indicated by the lower thick arrow proceeds from optical fiber 207 to optical fiber 201 is rotated counter clockwise relative to the proceeding direction by the Faraday rotator. Therefore the polarization centers of the ordinary and extraordinary beams neither coincide nor enter the optical fiber. This isolator is detailed in Patent Publication No. Sho-58-28561. This technique has not been able to be used to produce a circulator because the return paths don't coalesce for the different polarization states, and significant distortion would be introduced to the beam if a displacement comparable to a fiber width (125 micron) were to be achieved to allow the capture by a third fiber.
It is also possible to provide a circulator based only on polarization walk-off plates and Faraday rotation elements. In each case the separation and recombination of polarization is achieved by passing a substantially collimated beam through a polarization selective element. This form of polarization splitting is shown in FIG. 3. Light proceeding from fiber 301 is collimated by a lens 302 and one polarization state is displaced relative to a second polarization state by a doubly birefringent crystal 303 by an amount exceeding the beam width. Each of the beams can be focused with separate lenses 304 and 305 into fibers 306 and 307 respectively. Details of an implementation utilizing this principle for achieving the functionality of an optical circulator are described in Patent Publication 0 491 607 A2. The major deficiency of this implementation is the very long optical path lengths necessitated, Optical circulators which are based upon the use of walkoff plates (birefringent plates which laterally displace one polarization state relative to the other polarization state require long lengths of birefringent crystal to achieve a suitable walk off to allow the return light to be captured in a different beam. As such the devices can be bulky and difficult to ensure environmental insensitivity. In addition the large optical distances which the beam has to travel mean that the coupling losses can increase.
Another class of circulators employs a non reciprocal phase shift in an interferometric arrangement (Mach Zehnder). It is not however able to achieve very high isolation of the return light (typically 30 dB) Such an implementation is described in PCT Patent Application PCTAU9300146.
Another class of nonreciprocal devices has used expansion of the core size of a fiber to allow light to travel a significant distance through a walkoff plate or polariser and Faraday rotator element without incurring large coupling losses due to the diffraction effects of light. A polarization independent isolator has been constructed using this technique, although losses are still too high for many applications. The return light of this device is however lost into the cladding of the fiber and not able to be separately routed. Although it could in principle be possible to produce a circulator using this technique, the extra length and complexity that would be involved would make it very difficult to achieve low losses.
It is desired to provide a device for achieving substantially nonreciprocal routing of light which at least partly overcomes the deficiencies of the prior art.